The present invention relates to a frequency carrier allocation method for a cellular system using the FDD (Frequency Division Duplex) scheme of performing two-way communication between a base station and a mobile station by using different frequency carriers in the uplink and downlink directions and, more particularly, to a frequency carrier allocation method in a case wherein traffics in the uplink and downlink directions become asymmetrical to provide multimedia communication services or a cellular system in which the transmission efficiencies based on frequency carriers in the uplink and downlink directions become asymmetrical.
In general, in a cellular system using the FDD scheme, transmission and reception are simultaneously performed between a base station and a mobile station by using different frequency carriers in the uplink direction, in which signals are transmitted from the mobile station to the base station, and in the downlink direction, in which signals are transmitted from the base station to the mobile station.
In each cell (communication area) set for each base station, the allocated frequency carrier is divided with time or codes to virtually set many channels on one frequency carrier, thereby realizing communication between many mobile station and the base station with one frequency carrier.
In addition, each frequency carrier is simultaneously and repeatedly used in a plurality of cells to increase the number of frequency carriers that can be used in one cell.
A cellular system having a hierarchical structure is used. In this system, a plurality of micro-cells, each covering a range having a radius of about several hundred meters or less, are arranged in each macro-cell covering a range having a radius of about one kilometer or more. In a general hierarchical cellular system, macro-cells are set in correspondence with a plurality of macro-cell base stations, and micro-cells are set in correspondence with a plurality of micro-cell base stations in each macro-cell.
Each micro-cell is used to partly increase the traffic accommodation capacity in a place where many mobile stations gather and the entire traffic cannot be accommodated by one macro-cell or to provide high-quality communications to a place where radio waves from a macro-cell do not easily reach. If a frequency carrier having the same band width as that used by a macro-cell is allocated to a micro-cell, in particular, a traffic as large as that can be accommodated by the macro-cell can be accommodated by even the micro-cell having a small area. Therefore, even in an area where the traffic is very high, the traffic can be accommodated by using micro-cells.
In a cellular system having such a hierarchical structure, if the same frequency carrier is allocated to a macro-cell and a micro-cell, since the transmission power of the base station and a mobile station in the macro-cell is higher than that of the base station and a mobile station in the micro-cell, the macro-cell causes strong interference with the micro-cell. For this reason, different frequency carriers are used in the macro-cell and micro-cell.
FIG. 4 shows the schematic arrangement of each of transmission/reception apparatuses in macro-cell base stations and micro-cell base stations. FIG. 5 shows the schematic arrangement of each of transmission/reception apparatuses in the mobile stations. As shown in FIG. 4, a plurality of reception circuits and a plurality of transmission circuits are connected to an antenna through a transmission/reception multiplexer. As shown in FIG. 5, in the mobile station, a pair of reception and transmission circuits are connected to an antenna through a transmission/reception multiplexer.
Referring to FIG. 4, in the base station, a reception section 440 includes three reception circuits, and a transmission section 450 also includes three transmission circuits. The three reception circuits of the reception section 440 and the three transmission circuits of the transmission section 450 are connected to an antenna 410 through a transmission/reception multiplexer 420. An interference wave power measuring circuit 430 is connected to the transmission/reception multiplexer 420 to measure the reception power of interference waves coming from neighboring mobile stations.
Referring to FIG. 5, in the mobile station, a pair of a reception circuit 540 and a transmission circuit 550 are connected to an antenna 510 through a transmission/reception multiplexer 520.
As described above, the base and mobile stations respectively use transmission/reception multiplexers 420 and 520 each used to separate transmission and reception signals having different frequencies. Since transmission and reception signals that are simultaneously transmitted/received greatly differ in level, they must be separated sufficiently.
For this reason, the frequency intervals between frequency carriers used in the uplink direction and frequency carriers used in the downlink direction must be sufficiently larger than those between frequency carriers used in the same communication direction. The FDD scheme therefore uses two frequency bands separated from each other by a protective band width which is the frequency interval required to separate transmission and reception signals in the transmission/reception multiplexer 420. One of these frequency bands is exclusively allocated to communications in the downlink direction, and the other is exclusively allocated to communications in the uplink direction.
As described above, since different frequency carriers are allocated to macro-cells and micro-cells, a frequency carrier arrangement (allocation) like the one shown in FIG. 6 is used. In this frequency carrier arrangement, the numbers of frequency carriers used in the respective directions are constant.
In general, as described above, two frequency bands FB1 and FB2 have the same width, and the numbers of frequency carriers F11 to F16 and F21 to F26 in the downlink and uplink directions are the same. In the FDD scheme, if, therefore, the traffic ratios in the uplink and downlink directions differ, a frequency carrier shortage occurs in the direction in which the traffic is larger, while the frequency carriers cannot be fully used in the direction in which the traffic is smaller.
Assume that the traffic ratios in the uplink and downlink directions are the same. Even in this case, if the transmission efficiency based on the frequency carriers in one of the uplink and downlink directions is higher than that in the other direction, a frequency carrier shortage occurs in the direction in which the transmission efficiency is lower, while some of the frequency carriers in the opposite direction are left unused, and all the frequency carriers cannot be effectively used.
As a method of solving this problem, a method is disclosed in Japanese Patent Laid-Open No. 8-275230, in which the frequency carrier passband in the downlink direction is set to be larger than that in the uplink direction, and uplink frequency carriers and downlink frequency carriers are alternately arranged in the respective frequency bands.
According to this method, by allocating frequency carriers from the different frequency bands to each mobile station in the uplink and downlink directions, the utilization efficiency of the frequency bands can be increased while the required frequency interval between transmission and reception signals is ensured in each transmission/reception multiplexer even if the traffic in the downlink direction is larger than that in the uplink direction.
In this method, however, frequency carrier passbands in the uplink and downlink directions must be determined in advance by predicting traffic ratios and transmission efficiencies in the uplink and downlink directions. Although transmitters and receivers used in base stations and mobile stations must be designed in accordance with the respective passbands, the arrangements of the transmitter and receiver of each mobile station, in particular, are difficult to change.
For this reason, if the traffic ratios in the uplink and downlink directions change with changes in communication services provided by a system or the transmission efficiencies in the uplink and downlink directions change with advances in technology, the distribution of frequency bands cannot be easily changed in accordance with these changes, resulting in a deterioration in the utilization efficiency of the frequency bands.
There are therefore demands for a technique of distributing frequency resources in accordance with the needs in the uplink and downlink directions by only changing the arrangement of frequency carriers without changing the arrangement of the receiver of each mobile station even if the traffic ratios in the uplink and downlink directions are difficult to predict or the transmission efficiencies per frequency band in the uplink and downlink directions change with advances in technology in the future.
To solve this problem, frequency carriers may be asymmetrically arranged, as shown in FIGS. 7 and 8. As shown in FIG. 8, base stations are classified according to their positions into two groups BSG1 and BSG2. Referring to FIG. 8, white and black dots indicate the positions of the base stations; the white dots represent the base stations belonging to the group BSG1, and the black dots represent the base stations belonging to the group BSG2.
Each base station belonging to the group BSG1 selects frequency carriers in the downlink and uplink directions from frequency bands FB1 and FB2. For example, as shown in FIG. 7, such base stations respectively select frequency carriers F11 to F14 of the frequency band FB1 as frequency carriers in the downlink directions, and frequency carriers F25 and F26 of the frequency band FB2 as frequency carriers in the uplink direction.
Each base station belonging to the group BSG2 selects frequency carriers in the downlink and uplink directions from frequency bands different from those corresponding to the group BSG1. For example, as shown in FIG. 7, such base stations respectively select frequency carriers F21 to F24 of the frequency band FB2 as frequency carriers in the downlink direction, and frequency carriers F15 and F16 of the frequency band FB1 as frequency carriers in the uplink direction.
In this method, frequency carriers can be asymmetrically arranged in the uplink and downlink directions for the respective base stations and mobile stations while the frequency intervals between the frequency carriers in the uplink and downlink directions are separated by the protective band width or more.
When this method is applied to a cellular system having a hierarchical structure constituted by macro-cells and micro-cells in consideration of the necessity for arranging different frequency carriers between macro-cells and micro-cells, a frequency carrier arrangement like the one shown in FIG. 9 may be obtained.
In this case, macro-cell base station group 1 selects the frequency carriers F11 and F12 of the frequency band FB1 as frequency carriers in the uplink direction, and selects the frequency carrier F23 of the frequency band FB2 as a frequency carrier in the downlink direction. Macro-cell base station group 2 selects the frequency carriers F21 and F22 of the frequency band FB2 as frequency carriers in the uplink direction, and selects the frequency carrier F13 of the frequency band FB1 as a frequency carrier in the downlink direction.
Micro-cell base station group 1 selects the frequency carriers F14 and F15 of the frequency band FB1 as frequency carriers in the uplink direction, and selects the frequency carrier F26 of the frequency band FB2 as a frequency carrier in the downlink direction. Micro-cell base station group 2 selects the frequency carriers F24 and F25 of the frequency band FB2 as frequency carriers in the uplink direction, and selects the frequency carrier F16 of the frequency band FB1 as a frequency carrier in the downlink direction.
In general, when a cellular system uses the code division multiple access (CDMA) scheme as a radio access scheme, the same frequency carrier can be simultaneously set for all the cells. Therefore, in a cellular system having a hierarchical structure constituted by two levels corresponding to macro-cells and micro-cells, the set frequency carriers can be simultaneously used in all the cells on the respective levels.
According to the method in FIG. 9, which is associated with the frequency carrier arrangement for the hierarchical cellular system, frequency carriers can be simultaneously set and used only between base stations belonging to the same base station group. When, therefore, the CDMA scheme is used as a radio access scheme, the same frequency carrier can be simultaneously used only between base stations belonging to the same base station group on the respective levels, i.e., the macro- and micro-cell levels.
This means that when base stations are classified into two groups on each level as shown in FIG. 8, each frequency carrier can be simultaneously used only in half of the macro-cells or half of the micro-cells. As compared with the case wherein the CDMA scheme is used to allocate the same frequency carrier to all the cells, according to the method shown in FIG. 9, the number of frequency carriers that can be set for each cell is reduced to half, provided that the total number of frequency carriers remains the same.
In addition, in the method in FIG. 9, the traffic that can be accommodated by one frequency carrier does not increase much. According to the CDMA scheme, the traffic that can be accommodated by one frequency carrier is limited by the total amount of interference wave power received from communications between the base station and other mobile stations in the same cell and communications between the base stations and mobile stations in neighboring cells.
If the number of cells that use the same frequency carrier is reduced to half, the interference wave power from neighboring cells decreases to almost half. However, the interference wave power in the same cell remains unchanged. For this reason, the reduction ratio of interference wave power is lower than 50%, and an increase in accommodated traffic per frequency carrier is smaller than a two-fold. That is, the traffic that can be accommodated by one frequency carrier does not increase much.
According to the frequency carrier arrangement in the conventional hierarchical cellular system in FIG. 9, therefore, the traffic that can be accommodated by each cell decreases, which is given by the product of the number of frequency carriers per cell and the accommodated traffic per frequency carrier. As a result, the utilization efficiency of frequency carriers per cell decreases.